In the drilling and operation of oil wells, it is necessary to isolate the well in the event of a catastrophe occurring to the well which may lead to the uncontrolled release of the oil and/or gas from the well into the surrounding area. This isolation is provided by valves which are normally biased to the closed position but which are actively maintained open during the operation of the bore hold. Such valves are known as subsurface safety valves or SSSVs for short, and are located at a convenient location down the well. (Although the term "down" is used, some bore holes may have considerable lengths which are far from vertical, and may be substantially horizontal.)
Such SSSVs are typically flapper type valves which seal off the whole bore of the production tube and are arranged above a packer which seals the production tubing to the existing surrounding casing of the well. Conventionally, SSSVs are fitted by arranging them on the end of the joined production tubing and lowering the tubing in the well by connection of subsequent lengths of joined production tubing until the desired location for the SSSV is reached. The packer may be attached to the SSSV at the remote end of the tubing and installed and activated together with the SSSV or alternatively the packer may already be in place and the SSSV located above it.
An hydraulic control line is provided on the outside of the joined tubing which is used to activate the SSSV to maintain it in the open position for use. It will be appreciated that if pressure is lost for any reason, for example in the event of a disaster, then the SSSV will automatically close, closing the well and preventing the release of any of the well fluids.